The present invention relates to systems for matching an antenna to a radio at a carrier frequency and more particularly to systems for sensing and adjusting antenna matching in the presence of interfering signals.
Many aircraft radios include an adjustable, tuned, antenna coupling network for maximizing the signal coupled to the antenna. When it is desired to change the operating frequency of the radio, both the radio and the antenna coupler must be retuned.
The radio is retuned by changing the carrier frequency on which the radio transmits or receives. If the radio uses a tunable oscillator to generate the transmission carrier frequency, then the radio's transmission frequency is changed to a new frequency by changing the frequency of this oscillator. This has been done by mechanically changing the resonant frequency of a tuned capacitor/inductor (L/C) circuit which includes at least one capacitor or inductor whose value can be varied mechanically. Some newer radios use frequency synthesizers to generate their transmission carrier frequency. If a radio's synthesizer produces a single frequency at a time, then the synthesizer is commanded to begin producing the desired new transmission frequency in place of the old one. If the synthesizer is one which produces many frequencies simultaneously, then the frequency in use is changed. The reception carrier frequency of the radio is changed by changing the local oscillator frequency which controls what range of RF frequencies are down-converted to within the receiver's intermediate frequency (IF) bandwidth. The modulation is detected or demodulated to provide a baseband (or audio) signal. The operating RF carrier frequency is adjusted to the same frequency for both transmission and reception. This retuning of the radio RF carrier frequency in either the reception or transmission mode does not change the tuning of either the antenna or the antenna coupler by which the radio is connected to its antenna.
Where a tuned antenna coupler is included in the radio system, that coupler must be retuned separately from the changing of the radio's operating RF frequency if signals having the maximum available power are to be transmitted and received. The purpose of the tuned antenna coupler is to match the impedance of the antenna to that of the radio at the carrier frequency which is in use. This minimizes reflections and standing waves and maximizes the percentage of the available power which is transmitted or received. The process of retuning the coupler is often referred to as tuning the coupler's tuning network. This specification will refer to the antenna coupler as an antenna matching network and to the process of retuning it as adjusting its impedance in order to avoid confusion with the radio's system for controlling its operating frequency.
One system for adjusting the impedance of an antenna matching network comprises a feedback loop including that matching network, means for adjusting the impedance of that matching network, a sensor for providing a mismatch signal representative of the degree of mismatch of that matching network at the operating frequency during transmission and a matching adjustment processor responsive to the mismatch signal for controlling the means for adjusting the impedance of the matcing network.
In an aircraft having two such radio systems which operate in the same general frequency band, such as the 2-30 MHz RF band, there is a problem of interference between the different radio systems. This interference is a result of coupling between the antennas of the two radios. The matching network of the first radio cannot be adjusted while the second radio is transmitting or adjusting its coupler because of the presence of an excess signal in the first radio's matching network at the frequency of operation of the second radio. In the past, this problem has been avoided by using the radios alternately rather than simultaneously. A need has now developed to be able to use both radio systems at the same time.
It is desirable to have an antenna matching network adjustment system which can adjust the matching network of its radio independent of the operational use of other radios.